1. Field of the Invention
The present invention relates to a coordinate input apparatus and, more particularly, to a coordinate input apparatus for causing a plurality of vibration sensors arranged on a vibration transmission plate to detect a vibration transmitted from a vibration input pen and for detecting the position of the vibration input pen, thereby inputting coordinates.
2. Related Background Art
A conventional coordinate input apparatus is proposed wherein a vibration is input from a vibration input pen incorporating a piezoelectric element to a vibration transmission plate and is detected by a plurality of sensors arranged on the vibration transmission plate, and a vibration transmission time is measured, thereby detecting the coordinates of an input point. Such a coordinate input apparatus is disclosed in U.S. Pat. No. 4,931,965 filed by the present applicant. In this coordinate input apparatus, an end portion of the vibration transmission plate is supported by a vibration-isolating member so that an input vibration is not reflected by the end portion of the vibration transmission plate and the vibration sensor does not generate a detection error caused by the reflected wave of the input vibration.
Conventional vibration-isolating members are developed to reduce noise and are mainly classified into members for reducing noise transmitted in air and noise transmitted through a solid body. When a conventional vibration-isolating member is used for the above application purpose, the vibration-isolating member for reducing noise transmitted through a solid body is used. An example of this vibration-isolating member is a thin antivibration sheet shown in FIG. 10A or a paint.
Referring to FIG. 10A, a vibration transmission plate 80 is to be vibration-insulated by an antivibration sheet 81. The vibration transmission plate 81 comprises a metal plate (e.g., an aluminum plate), a resin plate, or a glass plate. With this arrangement, the antivibration sheet 81 is adhered to the vibrating vibration transmission plate 80 to reduce the vibration of the vibration transmission plate 80, thereby attenuating the vibration of the antivibration sheet 81 and reducing noise.
The conventional antivibration sheet 81, however, is designed to suppress the vibration of the vibration transmission plate 80 as a whole by being adhered to the entire surface of the vibration transmission plate 80. As described above, therefore, in order to suppress the reflection wave at the end portion of the vibration transmission plate 80, the reflection wave cannot be sufficiently attenuated even if the antivibration sheet 81 is adhered to the entire peripheral portion of the vibration transmission plate 80, as shown in FIG. 10B.
FIG. 10B is a view showing vibration propagation when the antivibration sheet 81 is adhered in the entire peripheral portion of the vibration transmission plate 80, and FIG. 10C is a sectional view showing a part of FIG. 10B and, more particularly, showing a state of the reflection waveform.
Referring to FIG. 10C, a waveform A represents a vibration propagating at a vibration start point as a contact point between a vibration input pen 3 and the vibration transmission plate 80, a waveform B represents a waveform reflected at an end face of the vibration transmission plate 80, and a waveform C represents a wave reflected at a boundary between the vibration transmission plate 80 and the antivibration sheet 81.
In the conventional antivibration sheet 81, a vibration-isolating effect is large for a vibration generated by the entire vibration plate (i.e., natural vibration including a resonant vibration). However, as shown in FIG. 10B, the vibration is increased in a region where the antivibration sheet 81 is not mounted, and a sufficient vibration-isolating effect for the vibration propagating from this region as a traveling wave cannot be obtained. For this reason, as shown in FIG. 10C, the vibration can be slightly attenuated in a portion D where the antivibration sheet 81 is mounted. However, the wave reflected by the end face of the vibration transmission plate 80 cannot be sufficiently suppressed.
A new wave C reflected by the boundary between the vibration transmission plate 80 and the antivibration sheet 81 is generated by mounting the conventional antivibration sheet 81. When the conventional antivibration sheet 81 is mounted in the entire peripheral portion of the vibration transmission plate 80 of the coordinate input apparatus utilizing an elastic wave, the above two reflected waves are generated. When a direct wave from a vibration source is to be detected, the reflected waves become noise, thereby degrading detection precision, resulting in inconvenience.
It is also proposed to mount a vibration-isolating member consisting of a material having a high acoustic impedance in a peripheral portion and to mount a vibration-isolating member consisting of a material having a low acoustic impedance inside. However, since the vibration-isolating members are arranged as a double structure in the widthwise direction, the entire structure of the vibration propagation body becomes bulky.